HISTORY OF FUNGICIDES
There are different types of Fungicides that are agents or chemicals and are used to prevent or eradicate fungal infections from plants or seeds. Most fungicides have low to moderate toxicity.
In 1802, lime sulfur was first used to control mildew on fruit trees. A few years later in 1807, Prevost used copper sulfate as a seed treatment to prevent the bunt of wheat. In the latter part of the century, Millardet, a professor of botany at Bordeaux University, observed that grapes treated with a mixture of copper sulfate and lime to make them unappetizing to travelers were free of downy mildew.
In 1885, he demonstrated the effectiveness of this “Bordeaux mixture” in controlling the downy mildew of grapes. Organomercurial fungicides were used as seed treatments to control the bunt of wheat in 1913.
The use of mercury and other heavy metals is now restricted by legislation due to animal toxicity. In the 1930s, organic fungicides called dithiocarbamates were developed. Dithiocarbamates such as maneb, ziram, and thiram were used to prevent foliar diseases of plants.
Although some of the original fungicides of this group are no longer available, dithiocarbamates are currently some of the most widely used fungicides around the world. By 1950, antibiotics such as streptomycin sulfate and cycloheximide were introduced to control bacterial or fungal diseases of plants.
From 1960 to 1970, widely used protectant fungicides such as captan and chlorothalonil were first used for foliar diseases of plants. One of the first systemic fungicide groups, the benzimidazoles, was introduced to combat a wide variety of diseases such as powdery mildew, stem rots, and leaf spot diseases around 1970.
Other systemic fungicides were introduced in the following decade. The SBIs (sterol biosynthesis inhibitors) were true systemic fungicides that controlled diseases such as powdery mildew and leaf spots. The acylalanines were the first systemic fungicides used to combat root rot diseases caused by species of Pythium and Phytophthora.
By the end of the 20th century, a new group of fungicides, the strobilurins, were introduced to prevent a long list of diseases such as mildews, leaf spots, and stem and root rots. The strobilurins are similar to antifungal compounds produced in nature by wood decay fungi. They are widely used in a row and specialty crops.
FUNGICIDE FORMULATIONS
Fungicide formulations consist of an active ingredient (a.i.) and inert ingredients and may contain wetting agents, emulsifiers, or stickers. Wetting agents and stickers help to distribute the product over the crop and slow the weathering process, respectively.
Prepackaged mixtures of two a.i.s may be used to broaden the spectrum of fungicidal activity or to prevent or slow the development of resistance. For example, mancozeb is often mixed with systemic fungicides at risk of resistance.
Different formulations of a fungicide may have different concentrations of the a.i. depending on its use and target organisms.
Wettable Powders (WP)
A wettable powder is a finely ground fungicide powder that does not dissolve when
added to water but remains suspended in the spray tank. Agitation of the spray solution is normally needed to prevent settling.
A disadvantage of a wettable powder is dust during measuring and weighing. Many of the older fungicides are formulated as wettable powder.
Water-Dispersable Granules (WDG)
Dispersable granules have larger particles than wettable powder. These particles break up rapidly when added to water and go into suspension. An advantage of a dispersible granule is less dust during measuring and weighing.
Water-Soluble Pouch (WSP)
Water-soluble pouches make mixing pesticides easier because a pre-measured amount of pesticide is sealed in a bag that dissolves in contact with water. These pouches are usually designed to be added to 50 to 100 gal of water.
An advantage of this formulation is that it minimizes exposure of the person mixing the pesticide. A disadvantage is that the pouches are preweighed for large volumes of spray and are not convenient for small quantities.
Flowable (F)
Flowable formulations have concentrated amounts of fungicide particles suspended in a liquid form. They are convenient because they are measured by volume rather than by weighing as in the dry formulations. A disadvantage of flowable formulations is that settling may occur if they are stored for long periods.
Emulsifiable Concentrate (EC)
An emulsifiable concentrate, like a flowable fungicide, is a liquid formulation. The active ingredient of this formulation is dissolved in an organic solvent. Emulsifiable concentrates are measured by volume.
Granule (G)
A granular formulation is made up of dry pellets or granules of a fungicide, often in low concentrations. These granules may break down or release the fungicide after coming into contact with water after application by a broadcast or drop spreader. Granular formulations are generally used for soilborne pathogens.
Dust (D)
Older fungicides such as sulfur are sometimes applied to the foliage of plants as fine dust. Dust formulations are often shaken or blown onto foliage and redistributed by dew or rainfall.
FUNGICIDE APPLICATION
Fungicides may be applied in many different ways. The application technique depends on the target pest, the crop, and available equipment. Most pesticides are mixed with water and applied by hand-held sprayers for small areas or by hydraulic sprayers or mist blowers for larger areas.
Hydraulic sprayers are used where larger volumes of water are used to apply pesticides to the canopy of crops. Hydraulic sprayers may use as much as 100 to 200 gal of water per acre to apply pesticides to the foliage of crops. Air blasts or mist blowers use much less water to apply the same amount of pesticide per acre.
An air blast sprayer in a nursery or orchard may use as little as 15 to 20 gal of water per acre to apply pesticides to the canopy of crops.
Fungicides are often applied as seed treatments or in-furrow applications to prevent seed rots and damping-off. Vegetable and field crop seed are often treated with fungicides to prevent seed rots caused by fungi such as species of Pythium and Fusarium species or damping-off caused by Rhizoctonia solani.
A brightly colored dye is added to the treatment to indicate that the seed has been treated with a fungicide and should not be used for food or animal feed.
Drench applications of pesticides are used in specialty crops for certain soilborne pathogens. Fungicides labeled for stem or root rot diseases are applied in large volumes of water to containers or soil beds, often through a proportioner connected to the hose or irrigation system.
Drench applications are intended to saturate the media or soil and may take the place of an irrigation cycle. Broadcast application of fungicide granules is used when drench applications are impractical. Granules may be applied over the top of containers, propagation beds, or landscape beds with a rotary spreader.
Drop spreaders are often used for the precise application of granules to turfgrass. Irrigation or rainfall is needed after granules are applied to release the active ingredient. Fungicide granules are sometimes incorporated into bulk soilless media prior to planting.
Granules are thoroughly mixed into the media along with other amendments such as fertilizer or lime. Fungicides incorporated into media generally combat damping-off and stem and root rot diseases. Types of fungicides are discussed below
TYPES OF FUNGICIDES
Fungicides are often grouped as contact or eradicate. Contact fungicides are most often applied as foliar sprays to protect above-ground plant parts from infection; they may also be used as seed treatments. Contact fungicides must be applied uniformly over the leaf surface.
As these fungicides are on the outer plant surface, they are subject to weathering and photodegradation. Contact fungicides do not protect new growth, so they must be applied frequently. Eradicate fungicides may be applied as seed treatments or on growing plants.
They are usually systemic and move upward through the plant in the xylem tissue. Eradicate fungicides, like contact fungicides, work best if they are in place prior to the infection; however, they will halt fungal growth if applied shortly after infection.
Systemic fungicides usually have one or more of the following characteristics: the ability to enter through roots or leaves, water solubility to enhance movement in the vascular system, and stability within the plant.
Systemic fungicides may move varying distances in plants after being applied. An advantage of systemic fungicides is that they provide control away from the site of application. They are inside the plant and are not affected by weathering.
Locally systemic (systemic) fungicides are the type of fungicides that are translaminar and will protect the undersides of leaves after being applied to the upper leaf surface. Locally systemic fungicides may provide protection for short distances on the leaf surface by vapor action.
Most truly systemic fungicides move upward in the apoplast (xylem) to protect new plant growth from infection. Movement in the apoplast is usually passive as the fungicide moves upward in the transpiration stream.
Arborists take advantage of this flow when they inject fungicides into the trunk of an elm to protect the tree from Dutch elm disease. An example of a fungicide that moves in the apoplast is azoxystrobin, a strobilurin fungicide.
Few systemic fungicides move downward in the symplast (phloem) after they are applied to foliage. Systemic fungicides that move in the symplast are transported in phloem sieve tubes to the root system along with carbohydrates manufactured during photosynthesis.
One of the few fungicides documented to move in the symplast is foretold.
| Fungicide Class | Common Name |
| Inorganic fungicides | |
| Sulfur | Sulfur, lime sulfur |
| Copper | Copper sulfate, copper pentahydrate, Bordeaux mixture |
| Dithiocarbamate | Mancozeb |
| Aromatic compounds | Pentachloronitrobenzene, chlorothalonil |
| Oxathiins | Carboxin, oxycarboxin |
| Benzimidazole | Benomyl, thiophanate methyl |
| Dicarboximide | Iprodione, vinclozolin |
| Acylalanin | Metalaxyl, mefenoxam |
| Organophosphate | Fosetyl-al S |
| Sterol biosynthesis inhibitor | Fenarimol, myclobutanil, propiconazole, triadimefon, triflumizole |
| Strobilurin | Azoxystrobin, kresoxim-methyl, trifloxystrobin |
| SAR stimulant | Acibenzolar-S-methyl |
FUNGICIDE RESISTANCE
If a fungicide is used continuously, there is always the possibility that certain individuals of a fungal population may become less sensitive to the fungicide. This decrease in sensitivity or resistance of a fungal population may be the result of a genetic mutation either present or induced in the population and the subsequent selection and multiplication of resistant individuals.
Environmental conditions, disease pressure, and fungicide application frequency affect resistance development. Fungicides that attack specific sites in the fungal cell may become vulnerable if the fungus becomes less sensitive to the fungicide with one mutation.
Fungi that produce a large number of spores are more likely to become resistant to fungicides. The best chance for resistance development occurs when thousands or millions of fungal spores representing numerous individuals are exposed to one fungicide continuously.
A single gene mutation in a few spores may lead to a lack of control if the individual spores are also highly pathogenic. Fungi that cause diseases of floral or vegetable crops in greenhouses such as powdery (Chapter 14) and downy mildew and botrytis blight have been reported to acquire resistance to some fungicides.
Cross-resistance may occur when a fungus becomes resistant to a particular fungicide active ingredient. For instance, Botrytis (gray mold) isolates that are insensitive to iprodione are often tolerant of the fungicide vinclozolin, which is in the same chemical class and targets the same site.
Also, powdery mildew that is resistant to benomyl is often resistant to thiophanate methyl, both benzimidazole fungicides. If a resistant strain of a fungus is present, it is often appropriate to choose a fungicide of a different chemical class.
The development of fungicide resistance can be slowed by the following strategies: •
- Using labeled rates for pesticide applications. The use of less-than-labeled rates may accelerate the development of resistance.
- Alternate or mix a fungicide of a different chemical class to slow resistance. Use fungicides vulnerable to resistance sparingly.
- Use fungicides as a part of an integrated disease management program that includes biological and cultural controls and host resistance when available.
NON-TARGET EFFECTS OF FUNGICIDES
The use or repeated use of certain fungicides may have unexpected results. The application of some fungicides may inhibit the growth of some fungi while having no effect on others.
Fungicides applied to the soil may reduce the growth of soil fungi such as Trichoderma species, which are important competitors of nutrients and potential parasites of plant pathogens.
The result is an increased incidence of diseases caused by plant pathogens that are tolerant of fungicide. Some SBI fungicides are closely related to plant growth regulators.
Use at higher than labeled rates or at shortened intervals may lead to shortened internodes and stunting of the turfgrass; use of high rates of SBI fungicides as seed treatments may reduce the growth of small grains such as barley or wheat. Broad-spectrum fungicides that inhibit the growth of saprophytic fungi in turf areas may lead to increased thatch layers.
References
- https://books.google.co.uk/books?id=04nSBQAAQBAJ
- https://books.google.co.in/books?id=04nSBQAAQBAJ
- https://books.google.com/books?id=e0T7CAAAQBAJ
- Fungicides in Crop Protection, 2nd Edition
- Fungicides: Classification, Role in Disease Management and …